System and method of improving sampling of wlan packet information to improve estimates of doppler frequency of a wlan positioning device

ABSTRACT

Systems and methods of improving sampling of WLAN packet information to improve estimates of Doppler frequency of a WLAN positioning device. A device estimates the position of itself. The device includes a WLAN radio module for receiving WLAN signals transmitted by WLAN APs in range of said device, extraction logic for extracting information from said received WLAN signals to identify the WLAN APs, and logic to cooperate with a WLAN-based positioning system to estimate the position of the device based at least in part on the extracted information identifying the WLAN APs in the range of said device. The WLAN radio module includes logic for measuring multiple received signal strength indicator (RSSI) values for sufficiently long WLAN packets to increase the sampled rate of RSSI of WLAN packets from WLAN APs and to thereby improve an estimate of the Doppler frequency of said device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) to thefollowing U.S. Provisional patent application, the contents of which isincorporated by reference herein:

-   -   U.S. Provisional Patent Application No. 60/819,182, entitled Use        of a Client-Side Receive-Only WLAN Device In a WLAN Positioning        System, filed on Jul. 7, 2006.

This application is related to the following U.S. Applications:

-   -   U.S. patent application Ser. No. 11/261,848, entitled Location        Beacon Database, filed on Oct. 28, 2005;    -   U.S. patent application Ser. No. 11/261,898, entitled Server for        Updating Location Beacon Database, filed on Oct. 28, 2005;    -   U.S. patent application Ser. No. 11/261,988, entitled        Location-Based Services that Choose Location Algorithms Based on        Number of Detected Access Points Within Range of User Device,        filed on Oct. 28, 2005;    -   U.S. patent application Ser. No. 11/261,987, entitled Method and        System for Building a Location Beacon Database, filed on Oct.        28, 2005;    -   U.S. patent application Ser. No. 11/365,540, entitled Encoding        and Compressing a WiFi Access Point Database, filed on Mar. 1,        2006;    -   U.S. patent application Ser. No. 11/562,514, entitled Location        Toolbar For Internet Search and Communication, filed on Nov. 22,        2006;    -   U.S. patent application Ser. No. 11/359,154, entitled Continuous        Data Optimization of Existing Access Points In Positioning        Systems, filed on Feb. 22, 2006;    -   U.S. patent application Ser. No. 11/359,144, entitled Continuous        Data Optimization of New Access Points In Positioning Systems,        filed on Feb. 22, 2006;    -   U.S. patent application Ser. No. 11/359,271, entitled Continuous        Data Optimization by Filtering and Positioning Systems, filed on        Feb. 22, 2006;    -   U.S. patent application Ser. No. 11/430,079, entitled Estimation        Of Speed and Direction of Travel In A WLAN Positioning System,        filed on May 8, 2006;    -   U.S. patent application Ser. No. 11/678,301, entitled Methods        and Systems For Estimating a User Position In a WLAN Position        System Based On User Assigned Access Point Locations, filed on        Feb. 23, 2007;    -   U.S. patent application Ser. No. 11/430,224, entitled        Calculation of Quality of WLAN Access Point Characterization for        Use In a WLAN Positioning System, filed on May 8, 2006;    -   U.S. patent application Ser. No. 11/430,222, entitled Estimation        of Position Using WLAN Access Point Radio Propagation        Characteristics In a WLAN Positioning System, filed on May 8,        2006;    -   U.S. patent application Ser. No. 11/429,862, entitled Estimation        of Speed of Travel Using the Dynamic Signal Strength Variation        of Multiple WLAN Access Points, filed on May 8, 2006;    -   U.S. patent application Ser. No. 11/430,064, entitled Estimation        of Speed and Direction of Travel In A WLAN Positioning System        Using Multiple Position Estimations, filed on May 8, 2006;    -   U.S. Provisional Patent Application No. 60/819,218, entitled        Methods and Systems for Using WLAN Positioning System in        Assisted GPS Systems, filed Jul. 7, 2006;    -   U.S. Provisional Patent Application No. 60/830,624, entitled        Methods and Systems for Using WLAN Positioning System in        Assisted GPS Systems, filed on Jul. 13, 2006;    -   U.S. Provisional Patent Application No. 60/821,479, entitled        WLAN Positioning System User Location Pulling Mode for Use in        Client-Server Systems, filed on Aug. 4, 2006;    -   U.S. patent application Ser. No. 11/625,450, entitled System and        Method for Estimating Positioning Error within a WLAN Based        Positioning System, filed on Jan. 22, 2007;    -   U.S. patent application Ser. No. 11/696,832, entitled Time        Difference of Arrival Based Estimation of Speed in a WLAN        Positioning System, filed on Apr. 5, 2007;    -   U.S. patent application Ser. No. 11/696,833, entitled Time        Difference of Arrival Based Estimation of Direction of Travel in        a WLAN Positioning System, filed on Apr. 5, 2007;    -   U.S. patent application Ser. No. TBA, entitled Method and System        for Employing a Dedicated Device for Position Estimation by a        WLAN Positioning System, filed on an even date herewith;    -   U.S. patent application Ser. No. TBA, entitled System and Method        of Gathering WLAN Packet Samples to Improve Position Estimates        of WLAN Positioning Device, filed on an even date herewith;    -   U.S. patent application Ser. No. TBA, entitled System and Method        of Gathering and Caching WLAN Packet Information to Improve        Position Estimates of a WLAN Positioning Device, filed on an        even date herewith; and    -   U.S. patent application Ser. No. TBA, entitled System and Method        of Passive and Active Scanning of WLAN-Enabled Access Points to        Estimate Position of a WLAN Positioning Device, filed on an even        date herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to WLAN-based positioning systems and,more specifically, to methods of using devices specifically dedicatedfor the collection of WLAN data for use by WLAN-based positioning systemto estimate a user's geographic position or optimize operation of theWLAN-enabled device or to gather more samples of WLAN signals to improveposition estimates.

2. Description of Related Art

In recent years the number of mobile computing devices has increaseddramatically, creating the need for more advanced mobile and wirelessservices. Mobile email, walkie-talkie services, multi-player gaming andcall following are examples of how new applications are emerging onmobile devices. In addition, users are beginning to demand/seekapplications that not only utilize their current location but also sharethat location information with others. Parents wish to keep track oftheir children, supervisors need to track the location of the company'sdelivery vehicles, and a business traveler looks to find the nearestpharmacy to pick up a prescription. All of these examples require theindividual to know their own current location or that of someone else.To date, we all rely on asking for directions, calling someone to asktheir whereabouts or having workers check-in from time to time withtheir position.

Location-based services are an emerging area of mobile applications thatleverages the ability of new devices to calculate their currentgeographic position and report that to a user or to a service. Someexamples of these services include local weather, traffic updates,driving directions, child trackers, buddy finders and urban conciergeservices. These new location sensitive devices rely on a variety oftechnologies that all use the same general concept. Using radio signalscoming from known reference points, these devices can mathematicallycalculate the user's position relative to these reference points. Eachof these approaches has its strengths and weaknesses based on the radiotechnology and the positioning algorithms they employ.

The Global Positioning System (GPS) operated by the US Governmentleverages dozens of orbiting satellites as reference points. Cell towertriangulation is another method used by wireless and cellular carriersto determine a user or device's location. Assisted GPS is another modelthat combines both GPS and cellular tower techniques to produce a moreaccurate and reliable location calculation for mobile users. In thismodel, the wireless network attempts to help GPS improve its signalreception by transmitting information about the clock offsets of the GPSsatellites and the general location of the user based on the location ofcell towers.

WLAN location system is a new positioning system, which uses WLAN accesspoints to produce location of mobile users. Metro wide WLAN basedpositioning systems have been explored by a couple of research labs. Themost important research efforts in this area have been conducted byPlaceLab (www.placelab.com, a project sponsored by Microsoft and Intel),University of California San Diego ActiveCampus project(ActiveCampus—Sustaining Educational Communities through MobileTechnology, technical report #CS2002-0714), and the MIT campus widelocation system. There is only one commercial metropolitan WLAN basedlocation system in the market at the time of this writing, and it isreferred to as WLAN positioning system (WPS) herein.

FIG. 1 depicts a WLAN positioning system based on WiFi signals. Thepositioning system includes positioning software 103 that resides on acomputing device 101. Throughout a particular target geographical area,there are fixed wireless access points 102 that transmit informationusing control/common channel signals. The client device monitors thesetransmissions. Each access point contains a unique hardware identifierknown as a MAC address. The client positioning software receivestransmissions from the 802.11 access points in range and calculates thegeographic location of the computing device using characteristics fromthe radio signals. Those characteristics include the unique identifierof the 802.11 access point, known as the MAC address, Time of Arrival(TOA), and the strengths of the signal reaching the client device. Theclient software compares the observed 802.11 access points with those inits reference database 104 of access points, which may or may not resideon the device as well. The reference database contains the calculatedgeographic locations and power profile of all the access points thegathering system has collected. The power profile may be generated froma collection of readings that represent the power of the signal fromvarious locations. Using these known locations, the client softwarecalculates the relative position of the user device 101 and determinesits geographic coordinates in the form of latitude and longitudereadings. Those readings are then fed to location-based applicationssuch as friend finders, local search web sites, fleet management systemsand E911 services.

Indoor or outdoor WLAN based positioning systems have leveraged existingoff-the-shelf WLAN cards without any modification other than to employthe logic to estimate position.

BRIEF SUMMARY OF THE INVENTION

The invention provides systems and methods of improving sampling of WLANpacket information to improve estimates of Doppler frequency of a WLANpositioning device.

Under one aspect of the invention, a device estimates the position ofitself in response to gathering wireless signal information from WLANaccess points (APs). The device includes a WLAN radio module forreceiving WLAN signals transmitted by WLAN APs in range of said device,extraction logic for extracting information from said received WLANsignals to identify the WLAN APs, and logic to cooperate with aWLAN-based positioning system to estimate the position of the devicebased at least in part on the extracted information identifying the WLANAPs in the range of said device. The WLAN radio module includes logicfor measuring multiple received signal strength indicator (RSSI) valuesfor sufficiently long WLAN packets to increase the sampled rate of RSSIof WLAN packets from WLAN APs and to thereby improve an estimate of theDoppler frequency of said device.

Under another aspect of the invention, the logic for measuring includeslogic to segment a WLAN packet and to measure a RSSI value for eachcorresponding segment.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a more complete understanding of various embodiments of the presentinvention, reference is now made to the following descriptions taken inconnection with the accompanying drawings in which:

FIG. 1 illustrates a high-level architecture of a WLAN positing system;

FIG. 2 illustrates a process by which information is extracted fromwireless data packets for transmission to a WLAN positioning system;

FIG. 3 illustrates information gathered by a dedicated device for use bya WLAN positioning system;

FIG. 4 illustrates a process by which packet errors are identified ininformation that is extracted from wireless data packets;

FIG. 5 illustrates the configuration of a dedicated device for gatheringWLAN signal information for a WLAN positioning system for estimation ofthe geographic position of the device; and

FIG. 6 illustrates the configuration of a device having a dedicatedprobe request transmitter for gathering WLAN signal information for aWLAN positioning system for estimation of the geographic position of thedevice.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention provide a dedicated device thatgathers wireless signal information for use by clients of a WLANpositioning system (herein “WPS dedicated device” or “dedicateddevice”). The dedicated device detects WLAN access points (APs) andprovides AP signal information to a WPS for use in estimating theposition, the velocity, and direction of travel of the device. Thededicated device for use with a WPS is designed to collect informationfrom as many detectable WLAN access point signals as possible, whilealso minimizing power consumption and device cost.

In one illustrative implementation, the receiver of a WPS dedicateddevice extracts MAC address from all the packets, including broadcast,multicast, unicast, and data packets, which are detected on the air bythe device's receiver to create a list of surrounding APs. That is, evenpackets with target addresses other than the dedicated device aredetected and used to estimate position of the dedicated device. In orderto detect WLAN APs surrounding a WPS user, source and destinationaddresses of packets, both are used. The IEEE 802.11 standard introducedtwo flags, To DS and From DS, which define the type of MAC addresses inthe header. These two flags are used to identify MAC address of AP inthe header. If there is no indicator of type of MAC addresses in theheader and which one belongs to an AP, all the MAC addresses areextracted and sent to WPS. Automatically, WPS only considers MACaddresses of APs, since only MAC addresses of APs exist in the databaseof APs. Note that each WLAN device including APs is assigned a uniqueMAC address. The received signal strength of all the packets can also bemeasured and reported to the WPS along with the type of the sourcepacket, as explained below, from which the MAC address is extracted.Preferred embodiments send the type of the packet along with the RSSI(Received Signal Strength Indicator) value to the WPS, because thetransmit power of some types of packets might not be known by the WPS.While the exact transmit power of some data packets might not be knownby the device's receiver, some signals are transmitted at a known powerlevel, e.g., beacon signals are transmitted at the maximum power at alltimes.

A typical WLAN-enabled device consists of a transmitter, a receiver, andlogic to support full duplex communication between the WLAN and theWLAN-enabled device. In a Wireless LAN the media is shared between allthe users and users contents to access the media and transmitinformation. Therefore, each user has to decode all the packets todetect and pick up packets destined to it. The logic of such a WLANcommunication device is configured to ignore certain types of signalsfrom WLAN access points (e.g. unicast signals intended for other WLANdevices). In contrast, the dedicated device of preferred embodimentsdoes not ignore any packet and extracts from each packet the MACaddress, RSSI, and other information useful to the WPS. The dedicateddevice does not need many of the transmitter and receiver functions thatare present in a typical WLAN communication device. The dedicateddevice, with logic dedicated solely for use with a WPS, can beconstructed more economically due to the reduced hardware and softwarerequirements. Further, the power consumption of such a device can belower than that of a full duplex device.

The embodiments of the invention described below may be used with theWi-Fi based location services and methods described in the relatedapplications listed above, which are herein incorporated by referenceand with the methods disclosed herein. Those applications taughtspecific ways to gather high quality location data for Wi-Fi accesspoints so that such data may be used to estimate the geographicposition, speed, and bearing of a Wi-Fi-enabled device utilizing suchservices and techniques of increasing the accuracy of the variousestimations.

Generally, as described in greater detail in the incorporatedapplications, in the WLAN positioning system, the user's WLAN-enabledmobile device scans for WLAN access points in range of the mobiledevice. The mobile device records information about the access points inrange (e.g., the access points' MAC addresses and received signalstrengths) and queries a database to receive the access points'geographic positions. This database can reside on the mobile device orit can reside on a central server and accessed remotely. Using thegathered information and the access point positions, the WLANpositioning system estimates the user's geographic position. Thetechniques of preferred embodiments enable additional signal informationto be gathered by the dedicated device. For example, packets that wouldbe ignored under conventional data communication technique are nowconsidered. Likewise, erroneous packets may be used. This additionalinformation enables the WPS to provide improved and/or more accurateposition, velocity, and bearing estimate, as described in detail below.

The WLAN positioning system can be implemented on a client-server basedarchitecture, in which the central database resides on a server andthere is a need for a connection to the server to determine a client'sposition. This connection can be implemented, for example, using a WLAN,or other radio signal methods, such as one or more of the various mobiletelephone radio standards. In addition, the positioning system can beimplemented on a client device with no server interaction, in whichcase, the various databases, tables and algorithms are located on thededicated device.

FIG. 2 illustrates a process by which information is extracted fromwireless packets for transmission to a WLAN positioning system. Thisprocess can be implemented using a WPS dedicated device without atransmitter. Upon receiving the radio signal from the AP, the physicallayer decodes the header of all of the received packets (step 201). Thelogic then extracts MAC addresses from the packets (step 202) andorganizes the MAC addresses based on the type of packet that wasreceived (step 203). For example, the packets may be a broadcast,multicast, or unicast packet. This table of gathered MAC addresses issent to the WPS as a list of detected APs (step 204), for use by the WPSto estimate the geographic position of the dedicated device using, forexample, the techniques disclosed in the above-incorporatedapplications.

In addition to extracting information from a more diverse selection ofpackets, the dedicated device can detect weak wireless signal by usingdiversity and combining multiple packets. Received packets at a weaksignal is subject to higher probability of error and if they do not passCRC check and at least an error is detected in them, using this method,the erroneous packets can be corrected by combing same information ofdifferent packets. The word “Diversity” here refers to a concept ofcombining different packets with some percentage of the same informationand reducing probability of error by combining them. For example thereceived packets may have only different sequence number and otherfields remain the same. Note that conventional definition of diversityis receiving multiple copies of the same transmitted packet/informationand combining them to improve the reception. There are differentknown/conventional methods of combining packets, like maximum ratiocombining, selective combining, or equal gain combining. Any combiningmethod can be used here to combine the common part of packets. Since theproposed method is based on combining different packets with somepercentage of common information, there is a need to identify packets,which can be combined. This is done by finding similarity of the packetsand if the similarity is above a threshold, the combining is examined.Thus, while an individual packet may have been unusable (e.g., becauseof CRC error), the diversity and combining techniques allow theseotherwise unusable packet to be used to improve position estimation(e.g., by providing more WLAN signal samples to the WPS).

This method increases receiver sensitivity by combining the headerinformation from multiple packets. By combining the header informationfrom multiple packets, the dedicated device can take advantage ofrepetitive information in the multiple packets and detect erroneousbits, correct them, and effectively increase the sensitivity of thereceiver of the dedicated device. Packets that are received at low powervalues are subject to a higher probability of error. In a typicalWLAN-enabled device, as received power decreases, more and more packetsare rejected at the physical layer because of errors in packets. Incontrast, when two or more packets originate from the same WLAN AP orare destined to the same WLAN-enabled device (which need not be thededicated device), the dedicated device combines the information in thepackets to detect errors and also correct errors present in the packets.

This technique enables detecting weaker signal from APs. In other words,it increases sensitivity by taking advantage of additional signalinformation that would otherwise be discarded by a typical WLAN-enableddevice. Increasing the number of detected APs improves the accuracy andcoverage of the position estimation. In order to detect APs, thededicated device need only process the packet headers to extract the MACaddress. If a series of packets that terminate at the same WLAN deviceoriginate from the same WLAN device, or are part of the same session,there is an extensive number of common fields in the headers. Byconsidering multiple packets with common fields, the dedicated devicecan help to identify the exact error(s) and potentially correct theerror(s) in a manner sufficient to at least use the packet informationfor position estimation.

There are many ways to perform data correction. In general, all of themethods use common information transmitted in the series of packets toidentify the errors and correct them. For example, by receiving twocopies of the same packet, the device can identify discrepancies betweenthe two and possibly correct those errors. Even if the errors cannot becorrected, identifying an erroneous part of a header can be used toensure that the MAC addresses are correct. Note that the headers of802.11 packets are protected using CRC (Cyclic Redundancy Check)techniques. Therefore, CRC checking used to detect errors.

In addition to the implementation described above, the WPS dedicateddevice includes the MAC address extracted from headers containingerrors, which cannot be corrected after applying any method, in the listof detected APs, along with a flag indicating that the addresses wereextracted from erroneous packets. Therefore, the list of detected APssent to the WPS includes a list of AP MAC addresses extracted frompackets without error and a list of AP MAC addresses extracted frompackets with error. Thus, the WPS can use the information associatedwith packets containing errors in such a way as to allow for the factthat the information may be incorrect. Increasing the number of detectedAPs improves the accuracy and coverage of the position estimation.

FIG. 3 illustrates information gathered by a WPS dedicated device foruse by a WLAN positioning system. An access point 301 transmits signalscontaining its MAC address. Mobile device 302 contains a WPS dedicateddevice. The dedicated device gathers information from APs in range,including access point 301, and generates a list of detected APs 303.This list is sent to a WPS 304. The list of detected APs 303 containsMAC addresses, and optionally, Received Signal Strength Indicationvalues. The entries in the list are categorized based on the type ofpacket that was the source of the MAC address (e.g., broadcast,multicast, or unicast) and whether or not the dedicated device detectedany CRC errors. This list is sent to a WLAN positioning system 304 toestimate the device's position.

FIG. 4 illustrates a process by which packet errors are identified ininformation that is extracted from WLAN packets. In the initial step,all detectable packets are captured, and each packet's header is decoded(step 401). As the MAC address of the AP originating the packet isextracted from the header, packets passing a CRC check are flagged witha correct flag (step 402), while packets failing a CRC check are flaggedwith an error flag (step 403). Next, all of the MAC addresses areorganized based on the type of packet that was the source of the MACaddress (e.g., broadcast, multicast, or unicast) and on whether or notthe packet passed the CRC check (step 404). Finally, the list is sent toa WLAN positioning system to estimate the device's position (step 405).

The WPS dedicated device only needs to read headers of the packets inorder to extract MAC address of APs. Therefore, the receiver of thededicated device needs only be equipped with corresponding physicallayer to decode headers. For example, because headers are transmitted atthe base rate, a WPS dedicated device need to decode the base rate. Thebase rate is a transmission rate that is common between differentversions of the WLAN standards. For example, the IEEE 802.11 standardprovides multiple options at the physical layer to extend the data rateto 11 Mbps for version ‘b’ and 54 Mbps for version ‘a’ and ‘g’ of thestandard. However, the header of these packets are always transmitted atthe base rate of 1 Mbps. A more accurate measurement of RSSI increasesthe accuracy of the estimation of position, velocity, and direction oftravel (using, for example, the techniques disclosed in the incorporatedapplications). However, measuring the RSSI of the packets is notessential for operation of the WPS. For example, the WPS can assume anominal power value for the packets without the power measurement andcan estimate the position of the mobile devices based on the nominalpower value.

Under an alternate embodiment, a receive-only WPS dedicated device isprovided that gathers WLAN access point signal information in a passivemanner (i.e., the device receives signals without the need to firsttransmit a request). In general, to enable a WPS to estimate theposition of a client device, the client device must detect WLAN accesspoints in the vicinity and, optionally, measure the RSSI valuesassociated with each detected AP. The receive-only WPS dedicated devicedetects WLAN access points as discussed in the incorporated applicationsand may also detect packets by sniffing packets originated by APsdestined to other WLAN-enabled devices or by receiving broadcast packetstransmitted by the WLAN access points. The sniffing and broadcasttechniques are particularly useful for receive-only devices as they lacka transmitter and cannot instigate an AP reply or response. Aftersniffing a packet or receiving a broadcast packet, the receive-only WPSdedicated device need only read the MAC address of the AP embedded inthe header of the packet and, optionally, measure the associated signalpower of the packet.

Power consumption of the WPS dedicated device can be minimized byminimizing duration of time that the receiver is active. The WPSdedicated device of preferred embodiments is in low-power consumptionmode (e.g. standby mode) and is activated when a packet is detected onthe wireless media. Moreover, it is only activated for the duration ofheader size. The WPS dedicated device only needs to receive the headerand decode it and may ignore rest of the packet. The WPS dedicateddevice can be equipped with a simple carrier sense module, which detectsany activity on the WLAN channels. The carrier sense module has lowpower consumption compared to the WLAN receiver module. The WLANreceiver module can be optimized for WPS to only extract the MACaddresses of detected APs and, optionally, to estimate the associatedRSSI value of the received signal. Upon receiving a request for aposition estimate, the device causes the carrier sense module to monitorWLAN channels for signal activity. When the carrier sense module detectsany activity, it changes the receiver module to active mode, which readsthe packet header and extracts the MAC addresses of WLAN APs.

In addition, the carrier sense module can be designed with passiveelectronic components to minimize power consumption. Furthermore, thecarrier sense module can be designed to operate at the RF band toeliminate the need for down conversions of the radio frequency to theprocessor frequency. Thus, any part of the receiver circuit that is notneeded for carrier sensing can remain in a low power consumption mode(for example, a standby mode) until needed. At which time, the carriersense module activates the receiver circuit upon detecting a packet onany channel.

FIG. 5 illustrates a configuration of a receive-only WPS dedicateddevice 500 for gathering WLAN signal information for a WLAN positioningsystem for estimation of the geographic position of the device. Thedevice 500 has an antenna 501 and RF modules 502, or parts of an RFmodule that are needed only to detect a carrier, and a carrier sensemodule 503 to detect any activity on the WLAN channels. The device 500also includes any other RF modules 504 that are required for WLAN signalprocessing, a baseband module 505, and a packet processing module 506that processes the packet header and extracts the MAC address andassociated RSSI values of the signals.

The RF modules 504 required for WLAN signal processing, the basebandmodule 505, and the packet processing module 506 are needed only upondetection of a WLAN signal. Thus, these modules are maintained in a lowpower consumption mode (e.g., a sleep mode). When the carrier sensemodule 503 detects any activity on a WLAN channel, the carrier sensemodule 503 changes all of the other modules to an active mode via powermode control connections 508. Also upon detection of a packet on thechannel, the carrier sense module 503 also routes the signal from the RFmodules 502 to the rest of the receiver modules via electronic switch507 to decode the header of the packet.

A “Beacon Detector” is one illustrative implementation of a receive-onlyWPS dedicated device. A beacon is a signal that is broadcast by WLANaccess points periodically and includes the MAC address of the accesspoint. Thus, a Beacon Detector is a simple receive-only device thatextracts the MAC address and, optionally, calculates the associated RSSIvalues, from the beacon signals.

In another embodiment of the invention, a WPS dedicated device isprovided that includes a transmitter and a receiver that are optimizedfor use in a WPS. In order to detect the WLAN access points in thevicinity and the associated RSSI values, the dedicated device activelyprobes WLAN APs by broadcasting a probe request message and receivingprobe response from the WLAN APs in range. While “probe response” and“probe request” refer to specific messages in the IEEE 802.11 standard,embodiments of the invention also encompass the general concept ofactive scanning for any WLAN access points. Active scanning is a quickand power efficient way of detecting WLAN APs because the duration ofscanning is short and the radio modules in the device are powered on foronly a short period of time, thereby consuming less power.

FIG. 6 illustrates one possible implementation of a WPS dedicated device600 in which the transmitter of the dedicated device is designed totransmit only the probe request message. This greatly simplifies theconfiguration of the transmitter. For example, the base band modules ofthe transmitter can be eliminated. Likewise, because the transmitter isdesigned to only transmit a pre-defined probe request packet, much ofthe software that would otherwise be required to control the transmittercan be eliminated. In this illustrative implementation, the proberequest packet is pre-generated and stored in a probe requesttransmitter module 601. Upon receiving a command to perform activescanning, the probe request packet can be fed to RF modules 602directly. The RF modules 602 transmit the probe request to surroundingWLAN APs via an antenna 603.

The WLAN APs in range respond with probe responses, which are passed bythe antenna 603 and RF modules 602 to a dedicated receiver module 604.The receiver module 604 extracts the MAC addresses from the proberesponses and calculates the associated RSSI values. The receiver module604 does not need to be able to decode any other message except theprobe responses. The IEEE 802.11 standard probe responses and proberequests are transmitted at the base rate. Thus, the probe requesttransmitter module 601, RF modules 602, and receiver 604 need onlysupport physical layer of probe request and probe response. Optionally,the receiver design can be extended to read all of the header files ofall the packets on the air, as described above, and compose a list ofthe MAC addressed and RSSI values of the detected APs. In such animplementation, the dedicated device has the dual benefits of being ableto gather information from surrounding WLAN AP on demand as well asbeing able to extract information from any detectable WLAN radio signalon the air. The device may then consult a local database to performposition estimation or use other forms of up-links to a remote database(e.g., cellular link).

The typical approach for known WPS-enabled devices is to leverageexisting WLAN transceivers embedded in (or added to) clients' devices(e.g., laptop, PDA, or cell phones). However, as described above, aWPS-enabled device of preferred embodiments does not require all of thefunctionality of a typical WLAN card in order to gather WLAN AP signalinformation. When a user's device is not equipped with a typical WLANcard, a WPS dedicated device can be added to the user device (laptop,cell phone, or PDA) to make it WPS-enabled. Thus, one potentialapplication of embodiments of the invention is to WPS-enable devicesthat are not equipped with a standard off-the-shelf WLAN transceiver.This can be achieved by adding a WPS dedicated device to user devices orby integrating a WPS dedicated device into general purpose integratedcircuits. For example, GPS devices, laptops, PDAs, and mobile telephonescan be WPS-enabled by including a WPS dedicated device into themicroprocessors and/or chipsets of these devices.

Embodiments of the invention address some of the main issues ofintegration with any mobile device general-purpose chipsets (e.g., GPSor microprocessors). Two general issues that apply in nearly allintegration cases are price and power consumption. In addition,embodiments of the invention also address specific issues that arise inthe integration of WLAN transceivers into specific devices. For example,WLAN transceivers can cause interference with GPS devices. Byeliminating the transmitter or limiting the use of the transmitter, bothas described above, potential interference is reduced or eliminated.Likewise, by employing the techniques disclosed herein, hardware costand power consumption can be reduced. Furthermore, when integrating aWPS dedicated device into a GPS receiver, the GPS and WPS dedicateddevice can share receiver hardware. For example, the receivers can sharethe antenna and/or local oscillators.

A WPS dedicated device can be purpose-built, or it can be built by usingan off-the-shelf WLAN card. In the later case, the unnecessary modulesand devices are not populated. In an alternate configuration, thehardware of a standard off-the-shelf WLAN card is fully populated, butunnecessary modules are disabled. In yet a further configuration, atypical off-the-shelf WLAN-enabled device is customized for use with aWPS. In such a case, the application programming interface of theoff-the-shelf interface is customized to perform the techniquesdescribed above. For example, as described below, the WLAN-enableddevice can be instructed to detect as many WLAN APs as possible, performextra signal measurements, and/or expose some of the signal measurementto the WPS to increase the accuracy of position, velocity, and directionof travel estimates.

Under another embodiment of the invention, the scanning process of aWLAN-enabled device can be optimized by combining passive and activescanning.

Passive and active scanning is part of the IEEE 802.11 standard. Hybridscanning is proposed by some WLAN device manufacturers, which isdescribed as an active scan followed by a passive scan of all thechannels or an active scan followed by a passive scan of the channeloperating on. Upon receiving a request to perform scanning, theWLAN-enabled device performs active scanning on all the channels,followed by passive scanning. Passive scanning can be performed on themost used channels. For example, for IEEE 802.11 use in the UnitedStates, passive scanning can be performed on channels one, six, andeleven, as these are the most popular channels. Furthermore, for theIEEE 802.11 standard implementation in the 2.4 GHz spectrum, twenty sixMHz channels are overlapped and separated only by 5 MHz. Therefore, morethan one channel can be scanned at a time for passive and activescanning. The RF hardware can be designed, using known methods, toenable the device to scan more than one channel at a time. For example,the RF filters can be made to have a wider bandwidth to enable thesignal of more than one channel to pass. If scanning process is dividedto steps, which are active scanning, passive scanning of the most usedWLAN channels and passive scanning of all the channels, scanning can befurther optimized based on the result of each step of scanning. In otherwords, scanning can be stopped after each step, if number of returnedAPs is sufficient to estimate location of the end user. There is abalance between the minimum number of required APs to estimate locationof an end user and power consumption. Increasing the minimum number ofrequired APs to estimate location of an end user, increases probabilityof scanning more steps and consume more power, but it increases overallaccuracy of the end user.

Under an extension of the above embodiment of the invention, the processof scanning of WLAN APs surrounding a WLAN-enabled device is optimizedbased on the status of the WLAN-enabled device. The goal is minimizingpower consumption due to scanning for localization of the WLAN-enableddevice by using information of the status of the WLAN-enabled device.For example, if WLAN-enabled device is in low-power consumption mode andnot associated with any AP, active scanning can be used which is shortand the device can go back to low power consumption mode quickly. IfWLAN-enabled device is in low-power consumption mode but it isassociated with an AP and goes to active mode periodically as part ofnormal operation of the device, the scanning for localization can beperformed at the same time that the WLAN-enabled device is activated forother reasons. Tying scanning for localization and other activities canalso be conditioned to a maximum time difference between the events. Inother words, if the time of request to scan for localization and otheractivities are more than a time threshold, they can scheduleindependently, otherwise they can be tied together and happen at thesame time. If a WLAN-enabled device is active and it is associated withan AP, active scanning followed by passive scanning can be performed,since the WLAN device is active anyway. The word “Association” isintroduced in the IEEE 802.11 standard and it means a logic connectionis established between the WLAN-enabled device and a WLAN AP.

Under another embodiment of the invention, WLAN device logs any detectedAP and time tags it for the last given period of time or up to themaximum number of recent detected WLAN access points, locally.

In the normal operation of a typical WLAN-enabled device, the devicedecodes packets to determine if the packets are intended for the deviceor decodes beacon broadcast signal; MAC address of surrounding APs canbe extracted from decoded packets or beacon signals, but they areignored or not saved for later use. By modifying the WLAN enabled devicethe list of detected APs along with the time of detection is savedlocally.

Then, at a future time, the cached information can be sent to a WPS,along with a time tag indicating the time of detection, in response to arequest for a position estimate. Storing history of detected APs is usedto increase accuracy of position, velocity, and direction of travelestimation and it also increases coverage in the case that the mostrecent scan does not return any AP in the database.

The cached information and associated time tags enable the WPS to takeadvantage of the history of the movement of the user, to track the user,and to increase the accuracy of position, velocity, and direction oftravel estimates. In addition, if the WLAN-enabled device cannot detectany APs when an AP scanning request is received, the cached signalinformation can be used to predict a later position of WPS-enableddevice.

In a navigation system, two classes of position estimation areintroduced, namely position of an end user can be estimated with orwithout history of movement. Without history is a single shot estimationand it is based on instantaneous received signal. It is a one timeestimation of position of end user. With history of movement, pasthistory of location of user and pattern of user movement is known to thesystem and current location of the user is not based solely oninstantaneous view of the signal, but instead also considers all theprevious history of movement. This is also called position estimationwith tracking. Accuracy of estimation in case of using history ofmovement is higher than one shot estimation. History of movement canalso be used to predict location of user at future time. Therefore, thepredicted location can be reported if there is no signal available tolocate user at the current time and improve coverage of positioningsystem.

Under another embodiment of the invention, multiple RSSI samples areprovide for each packet. Each packet is divided into multiple segmentsand RSSI is measured for each segment, separately. These RSSI values areused to estimate Doppler frequency reference to each AP using knownmethods. For example, the bandwidth of the power spectral density of thedistribution of power variation due to multi-path is equal to theDoppler frequency for Rayleigh and Rician distribution. As in known, inorder to estimate the bandwidth of power spectral density of a Rayleighor a Rician distribution, ideally, a number of power samples shouldsatisfy the Nyquist sampling theorem. Approaching the ideal Nyquistsampling rate increases the accuracy of Doppler estimation. Therefore,the power sampling rate can be a limiting factor on the maximum Dopplerfrequency that can be measured in this way. The WPS dedicated device andthe techniques described above enable an increase in the sampling rateof the power distribution by providing power readings for each packet itdetects and/or by providing multiple power readings for a given packet(e.g., one power reading associated with the first half of the packetand another power reading associated with the second half of thepacket). The RSSI values of each AP are considered separate from otherAP RSSI values.

There are extensive work and research in estimating Doppler frequencybased on power variation over time. No known WLAN based positioningsystem uses these methods to estimate Doppler frequency, because powersamples provided by a WLAN enabled device is not frequent enough toestimate higher frequencies. Therefore, this method with off-the-shelfWLAN card can be used to estimate very small Doppler frequencies or verysmall speeds, but in order to cover entire range of practical uservelocities, there is a need for more power samples per second.

As explained above, the various embodiments and implementations of theinvention can collect values of carrier frequency along with theinformation contained in the received packets and pass them to the WPSengine to estimate position, velocity, and bearing.

It will be appreciated that the scope of the present invention is notlimited to the above-described embodiments, but rather is defined by theappended claims, and these claims will encompass modifications of andimprovements to what has been described. For example, embodiments havebeen described as having a client-server architecture. However,embodiments of the invention can be implemented in the user's mobiledevice, in which the mobile device contains the access point locationdata and performs all techniques needed to determine the user'slocation.

1. A device for estimating the position of itself in response togathering wireless signal information from WLAN access points (APs),said device comprising: a WLAN radio module for receiving WLAN signalstransmitted by WLAN APs in range of said device; extraction logic forextracting information from said received WLAN signals to identify theWLAN APs; and logic to cooperate with a WLAN-based positioning system toestimate the position of the device based at least in part on theextracted information identifying the WLAN APs in the range of saiddevice; wherein said WLAN radio module includes logic for measuringmultiple received signal strength indicator (RSSI) values forsufficiently long WLAN packets to increase the sampled rate of RSSI ofWLAN packets from WLAN APs and to thereby improve an estimate of theDoppler frequency of said device.
 2. The device of claim 1 wherein saidlogic for measuring includes logic to segment a WLAN packet and tomeasure a RSSI value for each corresponding segment.